Computer systems store data to different types of storage media and devices. Such storage media and devices may be considered nonvolatile, and persistently store data when power to a computer system is turned off. An example of a nonvolatile storage device is a hard disk of a computer system. Storage devices may also include NAND flash memory and solid state disks (SSD). Storage media may include actual discs or platters that are accessed through the storage device. An operating system (OS) executing on a processor may request or perform actions, such as read and write, to particular locations on a storage medium.
Data written to and read from locations in these particular storage devices may be structured in blocks. Bits representing digital information (i.e., 1 or 0) may be grouped as data. In the storage devices, the bits may be stored in cells. Cells may be organized into pages. Therefore, a page is representative of the data. The size of a page typically is about 2,048 bytes for NAND flash memories; however, this is not typical for hard disk drives (HDD). In certain instances, the page may be a different size.
In some non-volatile memories, such as NAND-Flash, pages may be placed into erase blocks. An erase block typically includes about 64 pages, although in certain instances, an erase block may include a different number of pages. In such memories, it is typically required that all pages in a given erase block be erased together rather than individually.
Furthermore, in non-volatile memories such as NAND flash memory, it is typically required that pages are erased before they are written. Erased pages are also sometimes referred to as “blank” or “blank pages”. Thus, only blank pages can be written to. To write to the same page twice, the page is erased after the first write and before the second write. An exception to this rule is that bits in a written page may be toggled from “1” to “0” without an intermediate erase.
When an action such as a write is performed on a page of a storage device or storage medium, the entire erase block containing that page is first read into a temporary location, then the erase block is erased, and all the data is rewritten to the blank pages in the erase block, including the data from the temporary buffer for all but the requested page write, and the new data for the requested page write. Thus, a page write typically requires read, erase, and write operations on the entire erase block containing the page, which is relatively quite slow. The temporary locations may be in volatile memory of the computer system.
The number of erase cycles performed on erase blocks of memory like NAND flash memory may be limited. Typically, it is recommended that such erase actions are performed for no more than 100,000 cycles for each erase block.
Thus, in addition to degradation issues seen at erase blocks from multiple erase cycles, performance issues also exist when performing actions affecting entire erase blocks. Moving pages to and from erase blocks and temporary locations involves significant input/output (IO) traffic in a computer system and uses considerable processor (i.e., controller) resources.